1. Field of the Invention
The present invention relates to red light-emitting phosphorescent compounds (hereinafter, referred to simply to as ‘red phosphorescent compounds’) and organic electroluminescent (EL) devices using the same. More particularly, the present invention relates to red phosphorescent compounds, and organic electroluminescent devices comprising a laminate of an anode, a light-emitting layer and a cathode wherein one of the red phosphorescent compounds is used as a dopant of the light-emitting layer.
2. Discussion of the Related Art
With recent trends toward large-area displays, there has been increased demand for flat display devices that take up little space. In particular, technology of organic electroluminescent (EL) devices (also termed ‘organic light emitting diodes (OLEDs)’) as flat display devices has been rapidly developed. A variety of prototypes of organic electroluminescent (EL) devices have been reported to date.
When charge carriers are injected into an organic film formed between an electron injecting electrode (cathode) and a hole injecting electrode (anode) of an organic electroluminescent device, electrons combine with holes to create electron-hole pairs, which then decay to emit light. Organic electroluminescent devices have advantages in that they can be fabricated on flexible transparent substrates (e.g., plastic substrates) and can be operated at a voltage (e.g., 10V or below) lower than voltages required to operate plasma display panels (PDPs) and inorganic electroluminescent devices. Other advantages of organic electroluminescent devices are relatively low power consumption and excellent color representation. Further, since organic electroluminescent (EL) devices can emit light of three colors (i.e., green, blue and red), they have been the focus of intense interest lately as next-generation display devices capable of producing images of various colors. A general method for fabricating organic EL devices will be briefly explained below.
(1) First, a transparent substrate is covered with an anode material. Indium tin oxide (ITO) is generally used as the anode material.
(2) A hole injecting layer (HIL) is formed to a thickness of 10 to 30 nm on the anode. Copper (II) phthalocyanine (CuPc) is mainly used as a material of the hole injecting layer.
(3) A hole transport layer (HTL) is introduced into the resulting structure. The hole transport layer is formed by depositing 4,4′-bis[N-(1-naphthyl)-N-phenylamino]-biphenyl (NPB) to a thickness of about 30 to about 60 nm on the hole injecting layer.
(4) An organic light-emitting layer is formed on the hole transport layer. If necessary, a dopant may be added to a material for the organic light-emitting layer. For green light emission, tris(8-hydroxyquinoline)aluminum (Alq3) as a material for the organic light-emitting layer is deposited to a thickness of about 30 to about 60 nm on the hole transport layer, and N-methylquinacridone (MQD) is mainly used as the dopant.
(5) An electron transport layer (ETL) and an electron injecting layer (EIL) are sequentially formed on the organic light-emitting layer. Alternatively, an electron injecting/transport layer is formed on the organic light-emitting layer. In the case of green light emission, since Alq3 has excellent electron-transport ability, the formation of the electron injecting/transport layer may be unnecessary.
(6) A cathode material is coated on the electron injecting layer, and finally a passivation film is covered thereon.
The type of the organic electroluminescent devices (i.e. blue, green and red light-emitting devices) will be determined depending on the kind of materials for the light-emitting layer.
In the light-emitting layer, holes injected from the anode are recombined with electrons injected from the cathode to form excitons. Singlet excitons and triplet excitons are involved in the fluorescence and phosphorescence processes, respectively. Fluorescent materials using triplet excitons, which are involved in the phosphorescence process, whose probability of formation is 75%, exhibit high luminescence efficiency, as compared to fluorescent materials using singlet excitons whose probability of formation is 25%. In particular, the luminescence efficiency of red phosphorescent materials is considerably high, compared to that of fluorescent materials. Accordingly, a number of studies associated with the use of red phosphorescent materials in organic electroluminescent devices are being made to enhance the luminescence efficiency of the organic electroluminescent devices.
Phosphorescent materials for use in organic EL devices must satisfy the requirements of high luminescence efficiency, high color purity and long luminescence lifetime. As shown in FIG. 1, as the color purity of an organic EL device using a red phosphorescent material becomes higher (i.e. as the x-values on CIE chromaticity coordinates increase), the spectral luminous efficacy of the organic EL device decreases, making it difficult to achieve high luminescence efficiency of the organic EL device.
Thus, there is a demand to develop a red phosphorescent compound that exhibit desirable chromaticity coordinate characteristics (CIE color purity X≧0.65), high luminescence efficiency, and long luminescence lifetime.